687
1 INTRODUCTION
Failure of a ship’s own propulsion is a serious danger
not only for the ship not under command but also for
other vessels. From the definition of a ship underway,
you can define the drift process and then go to the
definition of a free-drift ship.
Influence of the vector of disturbed motion VZ on
the vessel VS results in the resultant drift vector ΔV as
shown in Figure 1a.
Figure 1. Ship drifting on the water surface
VZ external disturbances vector, Vd vector of object
movement over ground [Vd = f(VZ, A)], ΔV resultant
ship's drift vector, A extended section of the ship
When the ship loses the ability to move ahead the
vectors VZ and Vd are observed (Fig. 1b). During
external disturbances, a vessel or other free-floating
object loses its intended direction of movement. A
ship without its own propulsion cannot counteract
external disturbances, i.e. drift, thus becoming an
object not under command.
The value of the motion of a disabled ship depends
on:
disturbing parameters (VZ)
ship's geometric parameters (A).
2 COMPONENTS OF EXTERNAL DISTURBANCES
General division of ships in terms of external
disturbances according to geometric parameters:
A1 vessels with large windage areas more
sensitive to wind load influence
A2 vessels with low free board more
susceptible to current influence.
Processes of a Freely Drifting Vessel
M. Jurdziński
Gdynia Maritim
e University, Gdynia, Poland
ABSTRACT: The article describes the rules for planning a ship's navigation in the event of loss of propulsion. A
disabled ship drifting freely at sea is a potential danger to the crew and the marine environment. Lack of
propulsion means that the ship cannot give way to other ships/keep out of the way of another vessel. One of the
main elements of danger for a drifting ship is the possibility of grounding in restricted areas. The aim of the
article is to draw the attention of navigators to the dangers to navigation resulting from ships drifting without
their own propulsion, disabled ships.
http://www.transnav.eu
the
International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 14
Number 3
September 2020
DOI:
10.12716/1001.14.03.22
688
Figure 2. A1 and A2 Vessels and various FP/FW ratios
An example of a container ship surface is given for
comparison. The pressure from the wind on its
different side surfaces depending on the number of
layers of containers is surprisingly high. For example:
a container ship with a length of 400 m and a side
height of 35 m gives a lateral windage area of 14,000
m2. Table 1 gives wind load values on the ship’s hull .
Table 1. Wind load on the side area of 14,000 m
2
_______________________________________________
Wind speed [m/s] Pressure [t]
_______________________________________________
16 199
20 311
26 526
30 700
_______________________________________________
Drifts which are influenced by two major
elements, i.e. currents and winds of two different
ships have been analyzed here. Ships with A1
geometric structure are much more strongly
influenced by wind (passenger ships, ro-ro vessels,
etc.), whereas ships with A2 geometric structure (bulk
carriers, tankers, etc.) are affected more by the
influence of sea currents (tidal streams).
Ratio:
FP1/FW1 > 5/1 (A1)
FP2/FW2 > 1/5 (A2)
2.1 Components of external disturbances
The components of external disturbances are the
following:
wind vector VW, KW [knots, degrees]
wave vector hi, TF, KF [m, sec, degrees]
swell vector hM, TM, λp, KM [m, sec, m,
degrees]
ocean current vector Vp, Kp [knots, degrees]
tidal current vector Vpp, Kpp [knots, degrees]
wind current vector Vpw, Kpw [knots, degrees].
Additionally large vessels are influenced by a
Coriolis force.
2.2 Hull inclination of a drifting disabled ship
Location of the FP and FW gravity centers determines
the hull in motion in the process of free drift. Shifting
the FP point aft relative to the position closer to the
bow will cause the hull to turn windward, assuming
that the wind factor will be stronger than the current.
The vessel is said to tend to be windward.
The situation will be the opposite when the FP
point is closer to the forward part of the vessel than
the FW point the ship will be leeward. It will also
depend on the value of wind and current forces as
well as the Coriolis force.
The pattern of the wind direction acting on large
ships and the direction of drift also depend on the
ship's trim, usually small on large ships. Such
arrangements are shown in Figure 3a and 3b.
Figure 3. Position of hulls depending on wind when the
vessel is adrift
a) with trim 1/50 by the stern and wind speed 30 knots
b) under ballast with trim 1/50 by the stern at 50 knot wind
speed
The drift velocity of a tanker depends on the
loading condition of the vessel such as ballast or full
load.
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Figure 4. Graph of drift velocity of VLCC tanker loaded
and in ballast .
Figure 4 presents the drift velocity of a tanker in
ballast and loaded to capacity conditions as a function
of wind speed. In ballast condition vessels have a
larger windage area (FP) in relation to the submerged
part (FW).
Plan of the ship's drift close to dangers to
navigation is shown in Figure 10.
Figure 5. Situational plan of a disabled ship’s drift
The method of letting go the towing line onto a
ship in a drift is shown in Figure 6.
Figure 6. Chosen method of sending the tow onto the
disabled vessel
Three phases of sending the towline from the port
side of the tug H onto the vessel adrift where a
heaving line was sent to the disabled vessel and then
a towing line onto the vessel 1. Phase 2 making the
tug fast. The last phase is towing and agreeing on the
speed of the tow VS(H3).
Figure 7. In bad weather pouring wave quelling oil in lateral
drift
2.3 Calculation of the bollard pull needed to tow a
disabled vessel
The formula can be used to calculate the bollard pull
for a free drifting vessel:
(1)
M – required bollard pull in tonnes
D full displacement of towed vessel in tonnes
VH – tow speed in knots
B – breadth of towed vessel [m]
h height of the exposed transverse section of the
towed vessel including deck cargo, measured above
the waterline [m]
K a factor that reflects potential weather
K = 1.03.0 for exposed coastal tows
K = 0.752.0 for sheltered coastal tows
K = 0.51.5 for protected water tows
Another formula can also be used to calculate the
bollard pull for towing a disabled ship
( )
0.6
7.4M DWT= ×
(2)
In bad weather, ships over 100,000 (DWT) require
3 to 4 tugs of 3000 HP each .
The result of determining the bollard pull will
allow deciding on the number of tugs to stop the
ship's drifting and the method of holding the ship in
position when assistance of more tugs is required for
towing the ship to the place of refuge. In order to
reduce seaway parameters when the towing lines are
sent to the drifting vessel a method of wave quelling
oil spread on the sea surface may be used. See Figure
7.
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3 OPERATION DIAGRAM FOR PREPARING A
TOW PLAN FOR A DISABLED SHIP
Figure 8. Operation diagram for preparing a tow plan for a
disabled ship
4 MATHEMATICAL BASIS FOR A MODEL OF
THE SHIP’S DRIFT
The ship's drifting movement is of a stochastic nature,
it results mainly from the nature of the elements
disrupting the free drift of ships. The random nature
of the ship's drift can be described as the Makarov
process. This movement can be determined by the
formula (3) :
( )
,
x
d V x t dt d
ε
= +
(3)
d
xmotion of the drifting ship
xposition
ttime
d
ε - error of motion determination
If it is assumed that the drift results from the
impact of wind and current then formula (3) can be
represented as a formula (4) :
(
) (
)
( )
( )
11
0
00
x t x V t dt L t U t dt
−= +


∫∫
(4)
x(t)route for a specific moment t
x
0initial drift path
L(t)wind drift
U(t)sea current drift
The drift velocity can be obtained using the
following formula:
0
0
t
V V adt
= +
(5)
V
0is the initial drift velocity
Δt the time of drift
a is drift acceleration
The method of obtaining short distance of the drift
path can be calculated using the formula:
1
0
0
y x x Vdt= = +
(6)
V drift velocity
Figure 9 shows the model of the ship's drift under
the influence of sea current and wind. External
disturbances may also be affected by waves. An
important element of drift control is the use of
simulation. In such cases, a wide range of prognostic
information about hydro-meteorological factors
should be used whereas the details of drift model
description can be found in .
Figure 9. Pseudo ship’s drift
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5 FREELY DRIFTING VESSELS – CASES TO BE
SOLVED
5.1 Information on the weather condition in drift areas
Knowledge of weather factors is an important
element in safe towing of disabled ships. The
following is the minimum scope of meteorological
data in planning and implementing ship towing.
Standard information on facsimile maps
transmitted to ships in international navigation
includes such information as :
1 Analysis of sea surface weather.
2 Sea surface weather forecast.
3 Analysis of the windage area.
4 Analysis of sea waves.
5 Seaway forecast.
6 Analysis of surface water temperature.
7 Forecast of surface water temperature.
8 Information regarding ice cover and icebergs.
9 Significant weather condition.
10 Information on sea currents.
Information on the weather forecast onboard a
drifting ship is needed to reduce the risk of ship’s
damage when navigating in difficult areas. A high
risk of failure arises when a ship without shelter drifts
freely in the regions oil rigs and platforms. Similar
situation is observed when a drifting ship carrying
dangerous cargo approaches navigational obstacles.
The repair function is a probability function for the
duration on an engine failure.
( )
EF s
Ptt
>
(7)
( )
EF s
Ptt>
probability of an engine failure of ts hour
or longer
In case of such event on board a vessel there must
be another function to be used, i.e. repair function .
( )
ER s
Ptt>
(8)
( )
ER s
Ptt>
is the probability of the main engine
repair in t
s hour or longer
This assessment will allow you to make a decision
to call for external assistance to tow the ship away
from navigational dangers & navigational obstacles.
An example of a ship's drift near oil rigs is shown
in Figure 10.
Figure 10. Risk of drifting towards an oil field
5.2 Ship’s data recording
With the change of weather parameters the
parameters of the ship's drifts also change, therefore it
is necessary to monitor the ship's drift all the time.
A practical method of assessing the control of the
ship's position and the parameters of the ship's drift,
speed and direction over ground must be recorded in
the Ship Logbook.
Data Recording of ship’s event in the operating
process in accordance with SOLAS Convention 174
Reg. V/28 is the duty of the captain of a merchant
ship in international shipping. Data on the ship itself
and its operation can be divided into groups in
individual phases of navigation in various operating
conditions in a continuous system.
Entries include:
general information during normal ship operation
information on the description of special or
unusual events .
The daily record in the tow log should include
details such as:
1 Ship’s geographical position.
2 Weather conditions (sea state).
3 Tug(s) status.
4 Speed of the tow.
5 ETA at the port of refuge.
6 Technical condition of the towed ship.
7 Towing route.
These records can be made as follows:
manual entry in the Ship Logbook
constant automatic voyage data recording (VDR)
electronic recording in the central ship computer
(Log Towing ).
During free drift of a ship, records must reflect the
exact chronological sequence of events of the entire
ship failure process.
Figure 11 shows how to register a ship's drift path.
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Figure 11. Assessment of the ship's motion in the drift
Fi position of the ship
t
imoment of observation
A position of the bridge (antenna)
Observation of ship’s position at frequent intervals
allows to control the motion of the ship. On this basis,
further parameters of the ship's drift can be predicted
based on external disturbance conditions (prediction).
5.3 Predicting the track of a disabled vessel drifting in
tidal areas
In tidal regions, values of current change within an
hour, so a graphic picture of current changes over
time should be made. Additional drift resulting from
a wind of constant direction is added to get the plan
of the track of ship's drift. This example is shown in
Figure 12.
Figure 12. Way of predicting the ship's drift path in tidal
areas
5.4 Types of options of vessels’ failure in the drift
1 Damage to the ship's propulsion away from
navigational obstacles and away from assistance
(tugs), in deep waters without the possibility of
using anchors to control the drift.
2 Ship’s drift far from assistance in shallower waters
with the possibility of lowering the anchors to
control the drift.
3 1st and 2nd case of the ship’s drift with the
possibility of the propulsion system to become
operational within specified time.
4 Ship’s drift in shallow water in the vicinity of
navigational obstacles without the possibility of
repairing the propulsion system failure in a short
time which may result in risk of collision with the
navigational obstacle.
5 Ship’s drift near obstacles without tugs assistance
and without the possibility of repairing propulsion
systems in the vicinity of dangers to navigation.
6 Drifting ship in tow with a tug not powerful
enough; there is a drifting system near dangers to
navigation.
7 A drifting system that is at anchors and the tug
while waiting for assistance of greater towing
power.
8 Drifting tow with tugs unable to remain on the
planned route to the port of refuge in conditions
when wind and currents are increasing.
6 SUMMARY
Ship's drift may occur in bad weather as a result of
a limited propulsion system or a reduction in its
power.
Ship's drift may result from the ship's breaking off
the anchor in bad weather (after damage to the
anchor system).
Ship's drift is caused by damage to the steering
and/or propulsion system.
Ship’s voyage planning should also include
contingency plans showing how to act in case of
lost propulsion or steering system.
In any case there is a phase of rescue or towage to
ports of refuge when it is not possible to repair
damage to the ship's technical systems.
Rescue of a freely drifting ship is carried out with
the help of professional rescue teams or with the
participation of merchant ships (see Annex 1) .
Annex contains the ANNEX to the IMO MSC/Circ.
884 publication from 21 December 1998 under the
title Guidelines for Safe Ocean Towing. The annex
presents the method of towing a ship by a
commercial ship.
Towing plan based on the tow contract must be
prepared before towing.
The main points of the contract are the efficiency
of towing operation.
The towing plan covers a safe towing route and
speed to the port of refuge.
The weather is constantly monitored on the towing
route and the place of towing, as well as the ETA.
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An important element of towing planning is
establishing a method of communication and
cooperation regarding management of the tow.
An equally important element of the towing plan
is risk assessment of towing process.
The level of towing risk throughout the entire
towing operation depends on the following
elements :
1 Quality planning of the route of the tow.
2 Quality and reliability of communication in the
tow and outside the system.
3 Good supervision of the operation (operation
management).
4 Experience of tug crews.
5 Technical condition of tugs (towlines, winches,
engine power, etc.)
6 Lack of commercial pressure on the tugs crews
from outside.
7 Unexpected changes in the marine
environment, such as sudden changes in
weather (high seaways).
8 No unexpected increase in ship traffic along the
towing route.
9 Area free from pirate attacks.
ANNEX
Basic safety elements of the towed ship
Prior to towing operation, the towed vessel must
meet certain technical parameters in order to be
accepted on the tow in order to move from position A
to position B.
1 Training the crew in the safe operation of technical
systems related to the tow.
2 Before towing, the vessel must prepare lights and
signs prescribed for towing.
3 The vessel must have a specified draft (trim) for
the entire towing period.
4 The towed vessel must be watertight (when
making heavy weather).
5 The towed vessel must have a well secured cargo
(ballast water with no free space) etc.
6 The towed vessel must have safe stability in all
wave conditions and external disturbances.
7 Steering system, propeller (the rudder to be held in
the forward and aft position- midships). If
possible, the propeller should be secured against
turning.
8 Details on the technical and operational condition
of the ship should be collected on an ongoing basis
and ready to be reported.
9 Weather forecasts and sea state are to be
controlled. The captain must establish constant
contact with the mainland (shipowner, SAR, etc.).
10 Special Towing Booklet throughout the entire
towing passage must be kept.
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